To effect this object, it was of extreme importance to gain possession
of Stralsund, a town on the Baltic. Its excellent harbour, and the
short passage from it to the Swedish and Danish coasts, peculiarly
fitted it for a naval station in a war with these powers. This town,
the sixth of the Hanseatic League, enjoyed great privileges under the
Duke of Pomerania, and totally independent of Denmark, had taken no
share in the war. But neither its neutrality, nor its privileges, could
protect it against the encroachments of Wallenstein, when he had once
cast a longing look upon it.

The request he made, that Stralsund should receive an imperial garrison,
had been firmly and honourably rejected by the magistracy, who also
refused his cunningly demanded permission to march his troops through
the town, Wallenstein, therefore, now proposed to besiege it.

The independence of Stralsund, as securing the free navigation of the
Baltic, was equally important to the two Northern kings. A common
danger overcame at last the private jealousies which had long divided
these princes. In a treaty concluded at Copenhagen in 1628, they bound
themselves to assist Stralsund with their combined force, and to oppose
in common every foreign power which should appear in the Baltic with
hostile views. Christian IV. also threw a sufficient garrison into
Stralsund, and by his personal presence animated the courage of the
citizens. Some ships of war which Sigismund, King of Poland, had sent
to the assistance of the imperial general, were sunk by the Danish
fleet; and as Lubeck refused him the use of its shipping, this imperial
generalissimo of the sea had not even ships enough to blockade this
single harbour.